The present invention relates to a control system and control method of a combined system having a secondary battery and a generator and, more particularly, to allocation control of such a combined system.
In recent years, various research and development have been attempted to provide an electric vehicle installed with a secondary battery and a generator (electric power generator) as a combined power source for a drive motor and accessories with a view to improving an energy efficiency of the combined power source.
In such an electric vehicle, a fuel cell power generation system has been selected as an electric power generator to be installed.
In vehicles equipped with fuel cell power generation systems and secondary batteries, there are two types of running modes.
One typical example is a first mode wherein electric power output of the fuel cell power generation system is supplied to the drive motor, the accessories and the secondary battery, with the first mode being substantially continued even when the vehicle comes to a stop. This mode will be hereinafter referred to as a secondary battery-charging running mode.
The other mode is to supply both electric power outputs of the secondary battery and the electric power generator to the drive motor and the accessories, while the secondary battery being not charged by the electric power generator. This mode will be hereinafter referred to as a secondary battery-discharging running mode.
In such a vehicle having these two running modes, it is required to increase a total efficiency of the fuel cell power generation system and the secondary battery in respective running modes with a view to improving an energy consuming performance.
In view of the investigation by the present inventor, the secondary battery and the fuel cell power generation system have operating efficiencies of which characteristics are shown in FIG. 4.
Since the secondary battery has a Joule""s loss due to internal resistance, the operating efficiency will decrease as the amount of electric power output to be charged to or to be discharged from the secondary battery increases as shown by a curve a representing a case for discharging mode and a curve b representing another case for charging mode.
The fuel cell power generation system also has a similar characteristic in that it has the same Joule""s loss as that of the secondary battery and, in addition, it is suffered from the operating efficiency of a compressor for supplying oxygen to the fuel cell, with a resultant decrease in the operating efficiency with the increase in the amount of electric power generation output as shown by a curve c in FIG. 4.
For the above reasons, in the event that electric power is supplied to the drive motor and the accessories at a constant rate in the secondary battery-charging running mode, the amount of electric power output of the fuel cell power generation system is caused to increase with the increase in the amount of electric power to be charged to the secondary battery, resulting in a decreased total efficiency of the combined system.
Note should be undertaken here that the total efficiency is typically meant by a total operating efficiency that is obtained when the fuel cell power generation system is operated with fuel, at least a part of the electric power output of the fuel cell power generation system is used for charging the secondary battery, and sequentially the secondary battery is discharged.
In order to avoid a decrease in the total efficiency, it has heretofore been proposed to employ an operating method that decreases the amount of electric power to be charged to the secondary battery when demanded electric power is increased for the drive motor and the accessories.
FIG. 8 shows the relationship between an electric power generation efficiency of the fuel cell power generation system, a discharging efficiency of the secondary battery and the total efficiency with respect to electric power output of the fuel cell power generation system (abscissas of FIG. 8). Here, various curves are derived under a condition wherein demanded electric power for the drive motor and the accessories is 60 KW.
In the secondary battery-discharging running mode, since the sum of electric power output of the fuel cell power generation system and electric power discharged from the secondary battery is supplied to the drive motor and the accessories, the fuel cell power generation system will generate electric power output at a reduced power generation efficiency with the increase in the amount of electric power output produced by the fuel cell power generation system, as shown by a curve a in FIG. 8. In contrast, since the amount of electric power to be discharged from the secondary battery decreases with the increase in the amount of electric power output of the fuel cell power generation system, the discharging efficiency of the secondary battery increases as shown by a curve b in FIG. 8.
This causes the total efficiency of electric power outputs to be supplied from the fuel cell power generation system and the secondary battery to vary along a curve c shown in FIG. 8, with a resultant maximum total efficiency owing to particular relationships between the amount of electric power output of the fuel cell power generation system and the amount of electric power output to be discharged from the secondary battery.
Based on the above particular relationships, the amount of each electric power output of the secondary battery and the fuel cell power generation system is set to respective values for achieving the maximum total efficiency.
The charging efficiency of the secondary battery varies in the secondary battery-charging running mode as shown in FIG. 9 by controlling the secondary battery. Also, the charging efficiency of the secondary battery, which substantially corresponds to a more actual charging efficiency of the secondary battery when the secondary battery is charged in the combined system, is obtained by multiplying the power generation efficiency of the fuel cell power generation system and a charging efficiency of the secondary battery, both of which are obtained based on actual measurements or the like. The axis of abscissas represents the demanded electric power of the drive motor and the accessories.
In FIG. 9, a curve a indicates the charging efficiency of the secondary battery when the rate of charging the second battery is held constant, a curve b indicates the charging efficiency of the secondary battery when the rate of charging the secondary battery is varied in proportionate to the amount of electric power to be supplied to the drive motor and the accessories, and a curve c indicates the charging efficiency of the secondary battery wherein, when the charging efficiency of the secondary battery is lowered to a given level, the amount of electric power to be charged to the secondary battery is decreased to prevent the charging efficiency of the secondary battery from being lower than the given level.
Further, while the secondary battery is charged with generative power produced by the drive motor during deceleration of the vehicle, the power generation efficiency of the fuel cell power generation system has no relation to the secondary battery and the charging efficiency of the secondary battery to which the generative power is charged varies along a curve d shown in FIG. 9.
In the secondary battery-charging running mode, it has been attempted to combines various controls with a view to improving both the energy consuming performance of the vehicle and the performance of electric power.
By the way, a further research and development revealed that, even when the control is set in a manner discussed above so as to achieve the maximum total efficiency in the secondary battery-discharging running mode, it is difficult to obtain the maximum total efficiency throughout various running conditions of the vehicle. That is, in the above propsed method, the maximum total efficiency cannot be obtained throughout the whole operating conditions wherein electrical energy is produced by the fuel cell power generation system with the use of fuel and is charged to the secondary battery, which subsequently discharges electric power output.
Here, a reference will be given to FIG. 10, which shows various curves for total efficiencies which are obtained when the amount of electric power to be supplied to the drive motor and the accessories in the secondary battery-discharging running mode is 60 KW. The axis of abscissas represents the electric power output of the fuel cell power generation system and plural curves are plotted for each efficiency in cases where the secondary battery is charged in the secondary battery-charging running mode.
As will be apparent from FIG. 10, the total efficiency with respect to electric power output of the fuel cell power generation system (abscissas of FIG. 10) varies responsive to the efficiencies attained when the secondary battery is charged. For example, in the event that the efficiency is 70% when the secondary battery is charged, the maximum level of the total efficiency becomes 65% as shown by the curve B.
In the case that in a control system where a charging efficiency of the secondary battery is not considered, it is required for the charging efficiency of the secondary battery to reach 100% on a characteristic curve such that the maximum total efficiency is obtained when electric power output of the fuel cell power generation system is 20 KW and electric power output discharged from the secondary battery is 40 KW (i.e., 60 KWxe2x88x9220 KW).
However, assuming that the charging efficiency of the secondary battery is 70%, the maximum total efficiency is obtained at a point B in FIG. 10 under the conditions wherein electric power output of the fuel cell power generation system is 36 KW and electric power output discharged from the secondary battery is 24 KW (i.e., 60 KWxe2x88x9236 KW). In this event, the total efficiency at the point A, wherein electric power output of the fuel cell power generation system is 20 KW and electric power output discharged from the secondary battery is 40 KW, becomes smaller than the maximum total efficiency that would be expected.
Thus, the charging efficiency of the secondary battery varies in the secondary battery-charging running mode responsive to the running conditions of the vehicle and the total efficiency has the maximum point that varies responsive to the charging efficiency of the secondary battery in the secondary battery-discharging running mode.
It is therefore an object of the present invention to overcome the above situations investigated by the present inventor and to provide a control system and a control method of a combined system having a secondary battery and an electric power generator to provided the maximum total efficiency at substantially all times throughout operating conditions wherein, even when a charging efficiency of the secondary battery varies, the electric power generator generates electric energy that is charged to the secondary battery which then discharges electric power output.
In the present invention, a control system controls a combined system having a secondary battery and an electric power generator, and serving as a power supply for a load, wherein the secondary battery is charged by use of the electric power generator. The control system is provided with: a charging efficiency calculating section calculating a charging efficiency of the secondary battery in conjunction with an electric power output of the electric power generator; a total efficiency data calculating section calculating total efficiency data in response to a demanded electric power of the load; and an electric power supply controlling section controlling allocation of an electric power output of the secondary battery and an electric power output of the electric power generator both to be supplied to the load. The electric power supply controlling section sets an electric power output of the secondary battery and an electric power output of the electric power generator both to be supplied to the load on the basis of the charging efficiency and the total efficiency data.
In other words, a control system controlling such a combined system of the present invention is provided with: means for calculating a charging efficiency of the secondary battery in conjunction with an electric power output of the electric power generator; means for calculating total efficiency data in response to a demanded electric power of the load; and means for controlling allocation of an electric power output of the secondary battery and an electric power output of the electric power generator both to be supplied to the load, while setting an electric power output of the secondary battery and an electric power output of the electric power generator both to be supplied to the load on the basis of the charging efficiency and the total efficiency data.
Besides, a method of controlling such a combined system of the present invention calculates a charging efficiency of the secondary battery in conjunction with an electric power output of the electric power generator, calculates total efficiency data in response to a demanded electric power of the load, and controls allocation of an electric power output of the secondary battery and an electric power output of the electric power generator both to be supplied to the load, while setting an electric power output of the secondary battery and an electric power output of the electric power generator both to be supplied to the load on the basis of the charging efficiency and the total efficiency data.
Other and further features, advantages, and benefits of the present invention will become more apparent from the following description taken in conjunction with the following drawings.